Green tea is good for you, but only if drunk in moderation. While the polyphenols in green tea are credited with preventing heart disease and cancer, it seems they can cause liver and kidney damage if consumed in very large quantities, a review of studies into the toxicity of polyphenols has shown.

"People shouldn't be too alarmed by this, but those taking supplements may experience problems," says lead author Chung Yang of Rutgers, the State University of New Jersey.

He stresses that up to 10 small cups of green tea a day is fine. Problems are likely in people who take supplements, which can contain up to 50 times as much polyphenol as a single cup of tea.

Yang's review cites experiments in which rodents and dogs died from liver poisoning when given very large doses of polyphenols. He also reports cases of people with liver toxicity after overdosing on green-tea-based supplements. Their symptoms disappeared when they stopped taking the pills, only to return when they started taking them again (Chemical Research in Toxicology, vol 20, p 583).

http://www.newscientist.com/article/dn11786-green-tea-can-be-harmful-in-large-quantities.html